Polyester wire enamels composed of aromatic diacids, polyhydric alcohols and glycols are known from the disclosures in Precopio and Fox, U.S. Pat. No. 2,936,296; Meyer and Zalewski, U.S. Pat. No. 3,342,780; and Sheffer and Jordan, U.S. Pat. No. 3,141,859. These materials when prepared with or without the aid of cresylic acid solvents form unstable solutions in cresylic acids or in glycol ether type solvents, with crystalline poly(ethylene terephthalate) coming out of solution. Such enamels have a very short shelf life and, by virtue of their instability, preclude the preparation of commercially efficient high solids content wire coating solutions. U.S. Pat. No. 3,141,859 proposes to solve the problem by using expensive and toxic monochlorophenol to replace only part of the cresylic acid, which itself is corrosive, toxic and causes pollution problems. The above-mentioned patents are incorporated herein by reference.
It has now been found that polyester wire enamels can be prepared with or without the aid of a reaction solvent if 1,2-propylene glycol is used as the main glycol. Enamels can be made up from such polyesters with glycol ether or ester solvents, such as the methyl, ethyl or n-butyl ethers of ethylene or diethylene glycol to a high solids count (.gtoreq.50%) with no solution instability due to crystallization, even after standing for long times.
Moreover, this invention eliminates the need for cresylic acid type solvents and provides excellent continuous coatings for conductors, without any toxic effluents. For example polyester enamels based on 1,2-propylene glycol having glycol ethers or esters either bound into the resin during preparation, or added later by simple dilution produce smooth, flexible, continuous, and abrasion resistant coatings on electrical conductors, which are resistant to cut through, heat shock and electrical breakdown.